SUMMARY The rapid increase in developed countries over the past 30 years in esophageal adenocarcinoma (EAC) suggests strongly environmental influences. Barrett?s esophagus (EAC), a precursor lesion for EAC, is increasing and associated with alterations in esophageal microbiota. Obesity, another risk factor for EAC, is associated with an altered gut microbiome. Gut bacteria are able to promote inflammation and cancer through activation of myeloid cells that contribute to regeneration and immunosuppression. In preliminary studies, we have examined microbiota and myeloid cells in our L2- IL-1? mouse model of BE. This mouse model shows alterations in the GE junction microbiome at 12 months compared to WT mice, and housing in germ-free (GF) conditions leads to reductions in inflammation, metaplasia and dysplasia. Under SPF conditions, L2-IL-1? mice fed a high fat diet (HFD) show alterations in the microbiome and increased GE junction tumors. GEJ metaplasia and dysplasia are also accelerated by treatment with proton pump inhibitors (PPIs), which alters gastric microbiota, and by IL-8 expression, which mediates myeloid responses to microbes. Finally, we have developed tools (HDC-EGFP and HDC-DTR mice) that allow us to track or ablate immature myeloid cells/MDSCs that respond to microbes and promote gastrointestinal tumors. We propose 3 specific aims: (1) Define the role of gut microbiota in BE/EAC using the L2-IL-1b mouse model. The GE junction microbiota will be analyzed during disease progression using 16S rRNA sequencing, and the role in EAC determined by GF housing or antibiotic suppression, as well as by colonization with defined flora. (2) Do obesity and PPIs promote BE/EAC in part through microbiota? We will examine the effect of bacterial eradication on BE progression by HFD or PPIs. We will examine changes in microbiota in obese or slender BE patients. (3). Do bacteria induce BE/EAC primarily through myeloid cells? We will examine changes in myeloid cell trafficking to the GEJ in L2-IL-1? mice in response to microbiota and IL-8 using HDC-EGFP crosses, and assess their functional role through ablation and adoptive transfer. Taken together, these studies will define the potential role of microbiota in BE/EAC progression.